1. Field of the Invention
The disclosed embodiments are directed generally to fluid power systems employing over-center motors, and, in particular, to fail-safe operations that are configured to remove output torque from a fluid motor in the event of a malfunction in the associated system.
2. Description of the Related Art
In recent years, significant interest has been generated in hybrid vehicle technology as a way to improve fuel economy and reduce the environmental impact of the large number of vehicles in operation. The term hybrid is used in reference to vehicles employing two or more power sources to provide motive energy to the vehicle. For example, hybrid electric vehicles are currently available that employ an internal combustion engine and a generator, which generates electricity to be stored in a battery of storage cells. This stored power is then used, as necessary, to drive an electric motor coupled to the drive-train of the vehicle.
There is also interest in the development of hybrid hydraulic vehicles, due to the potential for greater fuel economy, and a lower environmental impact than hybrid electric vehicles. According to one configuration, a hybrid hydraulic vehicle employs an internal combustion engine (ICE) to drive a hydraulic pump, which pressurizes hydraulic fluid. The pressurized fluid is stored in an accumulator and later used to drive a hydraulic motor coupled to the drive wheels of the vehicle.
There is a class of hydraulic machines that may be employed in hybrid operation that includes a rotating barrel having a plurality of cylinders, and pistons reciprocating within the cylinders. The barrel is configured to rotate over a valve plate having inlet and outlet ports. The barrel rotates over the valve plate, and fluid passes into, and out of, the cylinders of the barrel. In a hydraulic pump, fluid is drawn into each cylinder from a low-pressure inlet port and forced out of the cylinder to a high-pressure outlet port. In a hydraulic motor, fluid from a high-pressure inlet enters each cylinder in turn and vents to a low-pressure outlet. Some machines, commonly referred to as pump/motors, are configured to operate as pumps or motors, according to how fluid is applied to the machine.
One type of pump/motor is a bent-axis pump/motor. The operation of a typical bent-axis pump/motor will be described with reference to its operation as a motor. Operation of such devices in “pump” mode will not be described inasmuch as such operation will be clear to one having ordinary skill in the art, in view of the following description.
FIGS. 1A-1C show sectional views of a portion of an over-center bent-axis pump/motor 100 according to known art. The motor 100 includes a valve plate 102 and a cylinder barrel 104, having a plurality of cylinders 106 within which pistons 108 travel reciprocally. Each of the pistons 108 engages a respective socket formed in a drive plate 110. The drive plate 110 is coupled to an output shaft 120 that is rotationally driven by the motor 100.
The cylinder barrel 104 is configured to rotate around a first axis A. The drive plate 110 rotates around an axis B, and is coupled to the rotating cylinder barrel 104 by a constant velocity joint 116 (only portions of which are shown in FIGS. 1A-1C). Accordingly, the cylinder barrel 104 and the drive plate 110 rotate at a common rate.
The valve plate 102, barrel 104, and pistons 108, which define axis A, are configured to rotate, i.e., change angle, with respect to the drive plate 110, which defines axis B, for the purpose of varying the displacement volume of the pump/motor 100. The degree of rotation of axis A away from a coaxial relationship with axis B is typically referred to as the stroke angle of the device. FIG. 1A shows the motor 100 stroked to a positive angle, FIG. 1B shows the motor 100 stroked to a zero angle, and FIG. 1C shows the motor 100 stroked to a negative angle.
FIGS. 2A-2C show the portion of the motor 100 viewed from the right as seen in FIGS. 1A-1C, respectively, with the valve plate 102 removed. The motor 100 shown in FIGS. 1A-1C is depicted as having cylinders directly opposite one another such that when one cylinder 106 is at top-dead-center (TDC), another will be at bottom-dead-center (BDC). This arrangement is pictured to provide a view of cylinders 106 at both TDC and BDC in the same figure. However, in practice (and as pictured in FIGS. 2A-2C), most hydraulic motors employ an odd number of cylinders, typically seven or nine.
In operation, as the piston cylinders 106 rotate around the axis A with respect to the valve plate 102, high-pressure fluid is valved into each cylinder 106 as it passes top-dead-center (TDC). The high-pressure fluid applies a driving force on the face of the piston 108, which acts axially on the piston 108 with respect to axis A. This force is transferred by the piston 108 to the drive plate 110. As each piston 108 passes bottom-dead-center (BDC), the fluid is vented from the cylinder 106, which allows the piston 108 to be pushed back into the cylinder as the barrel 104 rotates it back toward TDC. For the purpose of this discussion, it will be assumed that as the cylinders 106 rotate to the left of TDC or BDC, as viewed in FIGS. 2A-2C, they are pressurized at high pressure, while, as the cylinders rotate to the right of TDC or BDC, they are pressurized at low pressure.
Referring to FIGS. 1A and 2A, it may be seen that, with the motor at a positive-stroke angle, the pressure exerted on the pistons 108 in the cylinders 106 on the high-pressure side of the barrel 104 tends to drive the drive plate 110 in a counter-clockwise direction, as viewed in FIG. 2A. The amount of torque generated is directly related to the stroke angle of the motor. As the stroke angle diminishes toward zero, as shown in FIGS. 1B and 2B, the output torque also diminishes toward zero. However, as the motor passes a zero-stroke angle and moves to a negative-stroke angle, as shown in FIGS. 1C and 2C, the pressure will tend to drive the motor in the clockwise direction. Thus, the motor is reversible by passing zero, or “over-center” from a positive- to a negative-stroke angle, or vice-versa. If the motor 100 is caused to rotate against the applied torque, it will function as a pump, drawing fluid into the cylinders on the low-pressure side, and forcing the fluid out of the cylinders on the high-pressure side. This is in contrast to a positive-angle motor, in which the motor cannot be stroked to a negative angle. In such a motor, the polarity of the fluid pressure must be reversed in order to reverse the direction of torque.